An oscilloscope is an electronics test and measurement instrument used by an engineer to acquire and display a waveform of interest from a particular test point of a circuit under test. The earliest oscilloscopes had no triggering capability, and as a result were unable to produce a stable display of the waveform of interest. Triggering circuitry was added to early analog oscilloscopes to provide stability to the display, by always starting the drawing of the waveform on the oscilloscope screen in response to detection of a trigger event. Thus, occurrence of the trigger event would cause the waveform to be displayed in a stable fashion at the same place on the screen.
Modern real time digital storage oscilloscopes (DSOs) also employ trigger systems to achieve stable displays, but they operate in a substantially different way. A DSO continuously acquires sample points and stores them in a circularly arranged acquisition memory. That is, when the last memory location in the acquisition memory is filled, a memory pointer is reset to the top of the acquisition memory and the samples continue to be acquired and stored. The samples collected before receipt of a trigger event are known as pre-trigger data. The detection of a trigger event sets the “zero” time position for the acquisition, and following samples collected are known as post-trigger data. Another trigger will not be accepted until the acquisition of the post-trigger data is complete.
Stated another way, a trigger system employs technology that enables engineers to capture (trigger on) signals relative to a specific event in time. The technology is typically used for troubleshooting (i.e., debugging) high-speed digital circuits to identify events that cause improper operation of those circuits. A desired trigger event can be a result of unexpected analog or digital operation that occurs at a single point in time (an anomaly) or as a result of a logic sequence.
Modern DSOs have been enabled to trigger on a large variety of events. The most commonly selected trigger is Edge Trigger, which examines the signal under test for an occurrence of either positive-going or negative-going vertical transition. In addition, a list of advanced triggers known in the art comprises: Glitch, runt, pulse width, level, pattern, state, setup & hold violation, logic-qualified, time-out, predetermined window, a predetermined period, time-qualified transition, time-qualified pattern, and serial data triggering.
Sequential triggering is also known in the art and is useful when it is necessary to trigger the instrument from two different signals. In known sequential triggering systems, a main (or A) trigger event is selected from a menu of possible trigger events, and a delayed (or B) trigger event is engaged. Known oscilloscope sequential triggering systems provided no choice of trigger event for the B trigger. In sequential triggering the A-trigger arms the oscilloscope to trigger upon receipt of the B-trigger edge.
A form of sequential triggering is also known in the logic analyzer prior art. However, logic analyzer triggering is clock-based, not continuous. That is, a logic analyzer samples a trigger event with its sampling clock. In an oscilloscope, the trigger event is applied in analog form to a trigger comparator; there is no clock involved. A continuous-type trigger is required in an oscilloscope in order for the oscilloscope to precisely locate the trigger point in time with respect to the received data. Moreover, logic analyzers do not respond to “analog” triggers (i.e., those that require comparison to at least two thresholds, such as, rise time, fall time, window, runt, etc.). Finally, logic analyzers do not employ coupling modes other than DC coupling.
Unfortunately, even a DSO with all of the above-recited triggering functions cannot trigger on certain compound events as found in today's ever more complex circuitry.